Regarding the boat. Do not wait until you get to the bar/hotel Os Castanos to find out at what time it leaves. Much better for planning to call two, yes, there are 2, operators from the albergue in Armenteira.
Depending on which one you call, and depending on tides and the number of people they think will be getting on the boat, the operators will use a particular boat. This means that while one may set off at 3pm, the other may set off at 9am.
Phone numbers for both operators can be found at the Armenteira albergue, while the people at the Vila Nova de Arousa only sell tickets for one of them.
I had called the evening before, and knew there was a hoat that could leave in the morning. When I got to Vila Nova, I was told the boat only left mid-afternoon... Until I said "no, I spoke to an operator yesterday, and he said early morning". Like magic, the schedule changed, when the guy working with the albergue in VN realised he might lose a lot of business and all of a sudden found a boat that could leave in the morning, despite a lower tide.